As modern electronic circuit boards evolve toward increased circuit and component densities, thorough cleaning of the boards after soldering becomes more important. Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with a flux and thereafter passing this coated side of the board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes adhesion of the solder. Commonly used fluxes consist, for the most part, of rosin used alone or with activating additives such as amine hydrochlorides or oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the flux and flux residues are often removed from the board with an organic solvent. The requirements of such a solvent are stringent: a solvent should have a low boiling point, have low toxicity and exhibit high solvent power so that flux and flux residues can be removed without damage to the substrate being cleaned.
Flammability and solvent power characteristics can often be adjusted by preparing mixtures of solvents; mixtures, however, are often unsatisfactory because they fractionate to an undesirable degree if they are boiled. Such mixtures also fractionate during recovery, making it difficult to reuse a solvent mixture with the original composition without reformulation.
On the other hand, azeotropic mixtures, with their constant boiling and constant composition characteristics, have been found to be very useful. Azeotropic mixtures exhibit either a maximum or minimum boiling point and do not fractionate upon boiling. These characteristics are also important in the use of the solvent compositions to remove solder fluxes and flux residues from printed circuit boards. Preferential evaporation of the more volatile components of the solvent mixtures, which would be the case if they were not azeotropes, or azeotrope-like, would result in mixtures with changed compositions which may have less desirable properties, such as lower solvency for rosin fluxes, less inertness toward the circuit board electrical components and increased flammability. Azeotropes or azeotrope-like compositions are also desirable in vapor degreasing operations where redistilled material is usually used for final rinse-cleaning. Thus, the vapor defluxing as well as the degreasing system acts as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is an azeotrope or is azeotrope-like, fractionation will occur and undesirable solvent distribution may act to upset the safety and effectiveness of the cleaning operation.
A number of fluorocarbon based azeotropic compositions have been discovered and in some cases used as solvents for the removal of solder fluxes and flux residues from printed circuit boards and for miscellaneous vapor degreasing applications. Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes and this obviously complicates the search for new azeotropic systems which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropes or azeotrope-like systems which have desirable solvency characteristics and particularly a greater versatility of solvency power.